It is known that high molecular weight polybutylene terephthalate resins are uniquely useful as the principal components of compositions for the production of molded articles by the blow mold, injection molding and extrusion methods; see Winfield U.S. Pat. No. 2,465,319. The compositions find principal use in the production of television and radio sets, housings for kitchen appliances and hand tools, bottles, and parts for electric motors and automobile engines. Much effort has been expended in improving these compositions to improve the impact resistance of articles molded therefrom; see Lane U.S. Pat. No. 4,034,013; Baron et al. U.S. Pat. Nos. 4,034,016 and 4,044,072; and Farnharm et al. U.S. Pat. No. 4,096,202. Efforts have also been made to improve these compositions for electrical insulating purposes, and Seydl et al. U.S. Pat. No. 3,937,757 discloses that the "tracking" resistance of articles molded from polybutylene terephthalate molding compositions is improved when the compositions have a content of 5% to 50% by weight of a polyolefin component, which can be polyethylene, a poly(ethylene-vinyl acetate), a poly (ethylene-acrylate), and mixtures thereof. "Tracking resistance" is the ability of a molded article to resist formation of conductive tracks of carbonized material when an electrical arc is established on the surface of an article. Abolins U.S. Pat. No. 4,013,613 discloses similar compositions which, however, require a reinforcing filler.
The discovery has now been made that polybutylene terephthalate molding compositions which contain a small amount of an ethylene-ethyl acrylate copolymer, an ethylene-vinyl acetate copolymer, or a mixture of the two copolymers, and optionally polyethylene, the total weight of said copolymer or copolymers and said polyethylene (when present) being less than 5% but more than 0.1% of the weight of said composition, provide molded articles which in preferred embodiments possess greatly improved impact resistance and lower specific gravity, without more than negligible sacrifice of other properties.
The polybutylene terephthalate which is the principal polymeric component in the compositions of the present invention can be any of the polymers of this group or mixtures thereof. The polybutylene terephthalate which is present in the compositions of the present invention is preferably of such high molecular weight that it has an intrinsic viscosity of at least 0.6 and preferably about 0.8 dl./g. measured in a mixture of phenol and sym-tetrachloroethane in 3:2 by weight ratio at 30.degree. C. and a maximum viscosity of about 1.5 dl./g. Polymers having an intrinsic viscosity in the range of 0.8 to 1.2 dl./g. are preferred for injection molding applications since these polymers provide molded articles of excellent toughness and impact resistance.
The ethylene-ethyl acrylate and ethylene-vinyl acetate copolymers which are present in the compositions of the present invention are composed of ethylene and ethyl acrylate (or vinyl acetate) units in 10:1 to 1:10 molar ratio and preferably in 3:1 to 1:3 molar ratio. The polymers have molecular weights in the range where they are spinnable on the one hand and extrudable at ordinary temperatures on the other hand, that is, molecular weights between about 100,000 and 1,000,000.
The optional polyethylene is likewise of high molecular weight, in the range of 50,000 to 500,000.
The compositions of the present invention can and advantageously do contain some or all of the supplementary components which are customarily present in thermoplastic polyester molding compositions.
Thus they can advantageously contain one or more fibrous reinforcing agents, for example asbestos fibers, graphite fibers, cotton, silk, metal "whiskers" and the like. Because of their ready availability and the excellent increase in impact resistance which they impart, glass fibers (chopped to about 1/4" or less) are preferred. In addition, the composition can contain one or more pigments, for example titanium dioxide and barium sulfate whites, ultramarine and phthalocyanine blues, chrom green, carbon black and nigrosine black; mineral fillers for example talc, mica and, preferably, clay; flame retardants such as decabromodiphenyl ether and the other fully brominated aryl compounds disclosed in U.S. Pat. No. 3,751,396 alone or in admixture with Sb.sub.2 O.sub.3 and similar compounds as disclosed in U.S. Pat. Nos. 3,833,685 and 4,035,333; blowing agents; mold release agents; and stabilizers.
With regard to the proportions of the supplementary additives, the amount of fibrous reinforcing agent which is present can be as little as 1% or as much as 60% or more of the weight of the composition, amounts in the range of 5% to 50% being preferred. The fire retardant (or combination of fire retardants) is preferably present in amount between 1% and 20% of the weight of the composition as larger amounts detract from the properties of the moldings, and less than 1% confers little or no benefit. The remaining additives are present in customary minor amount.
The compositions of the present invention can be prepared in any conventional manner as, for example, by dry mixing the components in granular or powder form. The mixture can be extruded in the form of strands which after cooling can be chopped or otherwise comminuted into granular or powder form. The last procedure is specially advantageous because the particles so produced are uniform and remain uniform in composition on storage and transportation, so that differential settling during transportation does not occur.
Shaped articles can be molded from the compositions by any of the techniques heretofore used for the purpose. Temperatures and molding cycles in general conform with those presently employed for the production of moldings from polybutylene terephthalate based compositions. The most suitable temperature in any instance, however, is best determined by laboratory trial as this temperature varies with the molecular weights of the particular polybutylene terephthalate resin (and other resins) which are present, the presence of cross linkages and branching components as is shown in Cohen et al., copending U.S. application Ser. No. 957,801, and the flow characteristics which are desired, taking into account the dimensions and complexity of the mold. In most instances the compositions can be injection molded in an injection molding machine at a cylinder temperature of 250.degree. C. and a mold temperature of 75.degree. C. with a 60-second molding cycle.